Enabling the attachment field can be done from the Theme Panel – Shop & Auctions Settings:
1. Adding a PDF/Doc link to a product (admin dashboard):
After enabling the Attachment field (link) theme option, go to Products and edit or add a product and search for the Input PDF attachment from the Advanced tab:
2. Adding a PDF/Doc link to a product (WCFM Marketplace/Dokan):
If the Attachment field (link) theme option is enabled by the admins, you will be able to see the attachment field (when logged in as a vendor):
After saving the product, a new tab will appear. It is named “Attachments” and will contain a button to open/download the PDF/Document;